build(deps): update rdoc requirement from 6.5.0 to 6.6.0

Updates the requirements on [rdoc](https://github.com/ruby/rdoc) to permit the latest version.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.5.0...v6.6.0)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile

@@ -5,6 +5,6 @@ gemspec
 gem("minitest", "5.20.0")
 gem("rake-compiler", "1.2.5")
 gem("rake-compiler-dock", "1.3.1")
-gem("rdoc", "6.5.0")
+gem("rdoc", "6.6.0")
 
 gem("ruby_memcheck", "2.2.1") if Gem::Platform.local.os == "linux"